®

PCM54/55

5

POWER SUPPLY SENSITIVITY

Changes in the DC power supplies will affect accuracy.

The PCM54 and PCM55 power supply sensitivity is shown

by Figure 2. Normally, regulated power supplies with 1% or

less ripple are recommended for use with the DAC. See also

Power Supply Connections paragraph in the Installation and

Operating Instructions section.

SETTLING TIME

Settling time is the total time (including slew time) required

for the output to settle within an error band around its final

value after a change in input (see Figure 3).

Settling times are specified to

±0.006% of FSR; one for a

large output voltage change of 3V and one for a 1LSB

change. The 1LSB change is measured at the major carry

(0111…11 to 10000.00), the point at which the worst-case

settling time occurs.

STABILITY WITH TIME AND TEMPERATURE

The parameters of a D/A converter designed for audio

applications should be stable over a relatively wide tempera-

ture range and over long periods of time to avoid undesirable

periodic readjustment. The most important parameters are

bipolar zero, differential linearity error, and total harmonic

distortion. Most of the offset and gain drift with temperature

or time is due to the drift of the internal reference zener

diode. The PCM54 and PCM55 are designed so that these

drifts are in opposite directions so that the bipolar zero

voltage is virtually unaffected by variations in the reference

voltage. Both DLE and THD are dependent upon the match-

the current-source transistors. The PCM54 and PCM55 were

designed so that any absolute shift in these components has

virtually no effect on DLE or THD. The resistors are made

of identical links of ultra-stable nichrome thin-film. The

current density in these resistors is very low to further

enhance their stability.

DYNAMIC RANGE

The dynamic range is a measure of the ratio of the smallest

signals the converter can produce to the full-scale range and

is usually expressed in decibels (dB). The theoretical dy-

namic range of a converter is approximately 6 x n, or about

96dB for a 16-bit converter. The actual, or useful, dynamic

range is limited by noise and linearity errors and is therefore

somewhat less than the theoretical limit. However, this does

point out that a resolution of at least 16 bits is required to

obtain a 90dB minimum dynamic range, regardless of the

accuracy of the converter. Another specification that is

useful for audio applications is total harmonic distortion.

TOTAL HARMONIC DISTORTION

THD is useful in audio applications and is a measure of the

magnitude and distribution of the linearity error, differential

linearity error, and noise as well as quantization error. To be

useful, THD should be specified for both high level and low

level input signals. This error is unadjustable and is the most

meaningful indicator of D/A converter accuracy for audio

applications.

The THD is defined as the ratio of the square root of the sum

of the squares of the values of the harmonics to the value of

the fundamental input frequency and is expressed in percent

or dB. The rms value of the PCM54/55 error referred to the

input can be shown to be:

(1)

where n is the number of samples in one cycle of any given

10.0

3.0

1.0

0.30

0.10

0.03

0.01

0.003

0.001

510

15

±V

0dB

–60dB

–20dB

FIGURE 2. Effects of

±V

ε

rms

i

n

LQ

n

ii

=+

=

1

1

2

[( )

( )]

FIGURE 3. Full-Scale Range Settling Time vs Accuracy.

1.0

0.3

0.1

0.03

0.01

0.003

0.001

0.01

0.1

1.0

10.0

Settling Time (

µs)

R

Current

Output

Mode

Voltage

Output

Mode